Emergency services calls may be placed for many reasons, including requesting police assistance, requesting fire-fighting assistance, or requesting emergency medical assistance. Emergency services calls may be placed by dialing a predetermined emergency telephone number. Several countries have established a unique emergency telephone number that may be dialed anywhere within that country to place an emergency services call.
In many cases, the emergency telephone number has fewer digits than telephone numbers used to place non-emergency calls. The use of fewer digits allows the emergency telephone number to be dialed more quickly and reduces the burden of having to remember a telephone number having more digits. As examples, the emergency telephone number in Japan is 119, and the emergency telephone number in the United Kingdom is 999. As another example, the emergency telephone number in the United States is 911. Other numbers may be used in other countries.
Different numbers may be provided for emergency calls from landline telephones versus wireless communications devices (WCDs). Alternatively, the same number may be provided to support emergency calls from both landline telephones and WCDs. In most parts of the United States, both landline telephones and WCDs, such as cell phones, may place emergency services calls by dialing the digits 9-1-1.
Despite advances in the general availability of emergency services, the ability of a particular wireless 9-1-1 caller to connect to a shared wireless network and ultimately to a public safety answering point (PSAP) remains dependent upon an availability of wireless capacity provided by local wireless service providers. There are many points in a typical wireless emergency call setup process that may act as a bottleneck to prevent a particular WCD from reaching a PSAP.
In a typical cellular wireless communication system, an area is divided geographically into a number of cells and cell sectors, each defined by a radio frequency (RF) radiation pattern from a respective base station antenna. The base station antennae in the cells may then be coupled with a base station controller, which may then be coupled with a switch or gateway that provides connectivity with a transport network such as the public switched telephone network (PSTN) or the Internet.
When a WCD, such as a cellular telephone or wirelessly-equipped computer, is positioned in a cell, the WCD communicates via an RF air interface with the base station antennae of a cell. Consequently, a communication path can be established between the WCD and the transport network, via the air interface, the base station, the base station controller, and the switch or gateway.
Further, in some wireless communication systems, multiple base stations are connected with a common base station controller, and multiple base stations are connected with a common switch or gateway. Each base station controller may then manage air interface resources for multiple wireless coverage areas (e.g., multiple cells and sectors) by performing functions such as assigning air interface traffic channels for use by WCDs in the coverage areas and orchestrating handoff of calls between coverage areas. And the switch and/or gateway, in turn, may control one or more base station controllers and may generally control wireless communications by performing functions such as receiving and processing call requests, instructing base station controllers when to assign traffic channels, paging WCDs, and managing handoff of calls between base station controllers.
In general, air interface communications in each sector (or other such coverage area) of a cellular wireless communication system can be encoded or carried in a manner that distinguishes the communications in that sector from communications in adjacent sectors. For example, in a Code Division Multiple Access (CDMA) system, each sector has a respective pseudo-random noise offset or “PN offset” that is used to encode or modulate air interface communications in the sector distinctly from those in adjacent sectors. Analogously, in other air interface protocols, communications in one sector may be distinguished from those in other sectors by frequency, time, and/or various other parameters.
Furthermore, each sector generally has a limited set of resources that can be allocated for use to serve WCDs in the sector. By way of example, each sector may define an air interface “access channel” on which WCDs can send “access probes” seeking to originate calls (e.g., voice calls, data sessions, and/or other “calls”) or seeking to register their presence in the sector. The access channel may itself have limited capacity. (Further, if multiple access channels are provided, they may cooperatively have limited capacity.) For instance, the access channel may define timeslots in which WCDs can send access probes and may thus have a limited number of such timeslots. If numerous WCDs begin sending access probes in the same sector around the same time, the access channel of the sector can become congested and can ultimately reach a point where any further attempts to send access probes would result in “access probe collisions” and thus call setup failures (blocked calls) or other registration failures.
As another example, each sector may define an air interface “paging channel” on which the serving base station can send access probe acknowledgements and traffic channel assignment messages to served WCDs. And the paging channel may similarly have limited capacity. (Further, if multiple access channels are provided, they may cooperatively have limited capacity.) For instance, the paging channel may similarly define timeslots in which the base station can send various messages to particular WCDs. If the base station has numerous such messages queued to send, however, the paging channel can become congested and can thereby delay call setup or the like.
As yet another example, each sector may have a limited amount of transmission power for base station transmissions to served WCDs. That transmission power may need to be divided between numerous base station transmissions, such as transmissions to specific WCDs and broadcast transmissions to WCDs generally. At some point, if there is too much demand for base station transmissions, the power level allocated to particular transmissions may decrease to a point that the quality of the transmissions may suffer. Additionally, cells in a wireless network may exhibit “cell breathing,” in which the more traffic a particular cell carries, the smaller the footprint radius becomes on both forward and reverse links that will support wireless traffic for that particular cell. The outer radius of the footprint, as measured from the base station antennae of the cell, is defined as the “cell edge” and determines the geographical service range of a cell at any given point in time (based on traffic load).
As still another example, each sector may have a limited number of traffic channels that its serving base station can assign at any given time (e.g., for concurrent use by numerous WCDs, or for other use). In CDMA, for instance, each traffic channel may be defined by encoding with a particular “Walsh code,” yet the sector may have a limited pool of such Walsh codes. Consequently, if more than that number of traffic channels are needed at a given time, the base station would need to reject additional requests for traffic channel assignment, thus blocking new calls. Alternatively, in time division multiplex systems, such as TDMA or 1xEV-DO (e.g., the 1xEV-DO forward link for instance), traffic channels may be defined through interleaved timeslots on the air interface. In that case, if more than a threshold number of air interface communications occur at once, the base station may be unable to serve any additional communications due to the absence of any additional timeslots. As a result, communications may be blocked or degraded.
And as yet another example, each sector may have a limited supply of hardware addresses, such as Medium Access Control identifiers (MAC IDs) that its serving base station may assign for use to identify WCDs operating in the sector. This is typically the case in systems operating according to the 1xEV-DO protocol for instance. If more than a threshold number of WCDs are operating in the sector at once, the base station may exhaust its supply of MAC IDs and may then be unable to serve any additional WCDs that seek call initiation. Consequently, when additional WCDs request call initiation, the base station may need to reject their requests, again resulting in blocked calls.